1. Field of Invention
The invention relates to a threshold current modifying device and a threshold current modifying method, and, in particular, to a threshold current modifying device and a threshold current modifying method, which are applicable to an optical disk drive.
2. Related Art
An electric device is sensitive to temperature such that the performance and accuracy of the electric device may be affected by temperatures surrounding the electric device. In order to control the electric device at different temperatures, a proper control module must be designed to modify the effects resulting from thermal factors.
In order to control the output power of the electric device at different temperatures, a proper control module is designed in the electric device. Herein, a power feedback signal, which is generated when the electric device generates the output power, is applied to control the operating current for driving the electric device to generate the output power at different powers.
Referring to FIG. 1A, when the temperature of the electric device is at a temperature T1, a relationship between the output power and the operating current of the electric device is represented as a line L1. In other words, when the operating temperature of the electric device is at the temperature T1 and the operating current is at a current I1, the electric device generates the output power at a power P1. If the operating current is at a current I2, the electric device generates the output power at a power P2. If the temperature of the electric device rises to a temperature T2, the control module increases the operating current according to the power feedback signal until the electric device generates the output power at the power P1. At the same time, the operating current is equal to current I1′. In addition, with the prior technique a line L2 represents a relationship between the output power and the operating current of the electric device at the temperature T2. In order to control the electric device to generate the output power at a power P2, the control module calculates the required current I2′ to drive the electric device to generate the output power at the power P2 according to the operating current variance (I1′-I1) which results from the temperature variance for the power P1, and a scale relationship between the power P1′ and the power P2 in lines L1 and L2. The calculation is shown as the following formula (1).
                              I          2                =                                                            P                2                                            P                1                                      ⁢                          (                                                I                  1                  ′                                -                                  I                  1                                            )                                +                      I            2                                              (        1        )            
In fact, when the temperature of the electric device is at a temperature T2, the output power and the operating current of the electric device are actually related by a line L3 rather than the line L2. Therefore, the calculated operating current, which is equal to a current I2, drives the electric device to generate the output power at the power P3. Thus, the electric device does not correctly generate the output power at the power P2 and the output power is not precisely controlled at the temperature T2.
Because the prior technique disregards the threshold currents (intersections of the line L1, line L3 and the current axis), an error between the power P2 and power P3 occurs. If the threshold current is ineffectual, the error caused by the prior technique is not obvious. In other words, when the temperature of the electric device is at the temperature T2, the actual line L3 is close to the line L2 referred by the prior technique. However, if the threshold current is effectual or the output power is to be controlled more precisely, the operating current calculated according to the prior technique may include error.
Taking an optical disk drive for example, an optical pickup head of the optical disk drive is easily affected by thermal factors and behaves such as the electric device described above. Referring to FIG. 1B and FIG. 1C, in the power control structure of the optical pickup head in the optical disk drive, an optical disk drive 1 includes an optical pickup head 10 and a power control module 15. The optical pickup head 10 includes a light-emitting module 11, a temperature measuring module 12, a light-sensing module 13, and a digital-to-analog converting module 14. The temperature measuring module 12 measures the temperature of the light-emitting module 11 to output a temperature signal 121. The light-sensing module 13 senses the output power of the light-emitting module 11 to generate a power feedback signal 131. The power control module 15 receives the temperature signal 121 and the power feedback signal 131 to generate a current control signal 151 in the above method. The digital-to-analog converting module 14 receives the current control signal 151 to generate an operating current 141. The operating current 141 drives the light-emitting module 11 to generate output powers P1 and P2. The output powers P1 and P2 are respectively a writing power Pw and a reading power Pr for accessing an optical disk. However, if the temperature of the light-emitting module 11 rises, only the reading power P1 is precisely controlled but the writing power Pw due to the disregarded threshold currents. Therefore, the optical pickup head 10 does not correctly access the optical disk, and, even more, the lifetime of the optical pickup head 10 and the endurance of the optical disk drive 1 are reduced.
It is therefore a subject of the invention to provide a threshold current modifying device and a threshold current modifying method, which calculate the threshold current of the electric device at different temperatures. Thus, the output power of the electric device can be precisely controlled.